Excimer lamps are capable of generating ultraviolet (“UV”) radiation with very high efficiency. For example, excimer lamps can generate radiation in the spectral region of between about 50 and 200 nanometers wavelength, commonly known as vacuum ultraviolet or “VUV” radiation, with high efficiency. In certain applications, for example, general lighting, apparatus for reducing contaminants in a fluid stream, such as exhaust gases of a combustion engine and apparatus for germicidal use, it is desirable to convert the VUV radiation generated by such light sources to visible or longer wavelength UV radiation. Suitable phosphors are used in connection with such lamps to obtain the desired longer wavelength UV radiation.
A known light source for generating UV and VUV radiation is a mercury low pressure discharge lamp. A mercury discharge lamp, however, may be undesirable for many applications, because, if the lamp were to break, the mercury may be released, which may harm the environment as well as the components of an apparatus, such as a catalytic converter of automobile, in which the mercury lamp is included. In addition, mercury or other reactive chemicals in a mercury discharge lamp may degrade phosphors, such that mercury lamps that include phosphor for converting VUV radiation to longer wavelengths have a limited lifetime. The common general lighting fluorescent lamp incorporates phosphors to convert 254 nm wavelength UV radiation generated from the mercury to visible light.
One type of excimer lamp for generating VUV radiation is a dielectric barrier discharge (“DBD”) excimer lamp. The DBD excimer lamp typically includes a pair of electrodes coated with a dielectric and separated by a gas for producing excimer emissions, for example, noble gases, all of which are contained within a discharge tube or vessel. When it is desired that a DBD lamp produce radiation emissions in the visible or near UV spectral range, the dielectric barrier is coated with phosphor. DBD excimer lamps, however, generate a substantial amount of heat and also short wavelength radiation, each of which may cause degradation of phosphor. Also, DBD excimer lamps generate high energy ions or electrons that may bombard the phosphor in the lamp, which would result in degradation of the phosphor.
Another known excimer discharge lamp generates VUV radiation based on application of an electric field to a gas capable of forming excimers and providing free electrons in the gas. See U.S. Pat. No. 6,400,089, incorporated by reference herein. In such lamps, which are commonly known as corona discharge lamps, the electric field is typically configured to accelerate electrons between a first electrode and a counter electrode to at least the energy required to form excimers, but is configured so that in at least one region of the field, the field strength is below that required to substantially ionize the gas. It is also known to pulse the potential applied between the two electrodes in such excimer lamps for creating the electric field, so as to improve the efficiency of the lamp, while substantially avoiding harmful arcing within the discharge vessel. See U.S. Pat. No. 7,199,374 (“the '374 Patent”), incorporated by reference herein. When phosphor is included within the electric field region of such excimer lamps to produce longer wavelength radiation, however, the phosphor may degrade.
Also known is an electrodeless excimer lamp, which generates VUV radiation based on introduction of energetic electrons into a gas capable of forming excimers, so as to provide energetic free electrons in the gas. See U.S. Pat. No. 6,052,401 (“the '401 Patent”), incorporated by reference herein. In such lamps, which are commonly known as electron beam pumped lamps, high energy electrons, typically 10 to 20 kev, are injected through a thin ceramic membrane into the excimer forming gas. With this type of lamp as well, when a phosphor is included within the lamp to produce longer wavelength radiation, the phosphor may degrade.
Although coatings have been applied to a phosphors contained within devices such as plasma display panels for protecting the phosphor from degradation based on high energy ion or electron bombardment, see U.S. Pat. No. 7,223,482, the use of such coatings lowers the efficiencies such devices.
Therefore, there exists a need for a lamp for efficiently generating VUV radiation and converting the VUV radiation to longer wavelength radiation using phosphor, while avoiding degradation of the phosphor.